1. Field of Technology
This invention relates to an ink-jet recording device and, more particularly, to a droplet generator in a multi-nozzle continuous ink-jet apparatus which can eject ink droplets very steadily with high reliability and high maintainability.
2. Prior Art
The basic principle of operation of a multi-nozzle continuous ink-jet apparatus is explained below. Flying micro droplets are formed by compressing liquid in an ink tank by a pump or the like, letting liquid be ejected from an orifice, and applying vibrations to the ejected liquid by a piezoelectric element. Then the flying droplets are given charges by a charge electrode which is placed near the stream of droplets and have the controlled amounts of charge. The flying direction and the quantity of deflection of each charge-controlled droplet are controlled by a deflection electrode which is provided in the downstream side of the charge electrode. This controlling is done to form an image or pattern according to its information. Unwanted droplets that are not used for image or pattern formation are controlled to fly into a gutter which is provided in part of the flying path of the droplets. The liquid in the gutter is fed back to the ink tank for re-use.
In addition to this multi-nozzle continuous ink-jet type, there is a drop-on-demand ink-jet type which controls ejection of each droplet. This ink-jet type uses small ink chambers made of piezoelectric elements and deforms respective ink chambers by piezoelectric elements to eject droplets. Another well-known drop-on-demand ink-jet type provides a heater in each ink chamber, overheats liquid in the ink chamber by the heater to form a bubble, and pushes out liquid by the pressure of the bubble.
Contrarily, the continuous ink-jet apparatus need not control ejection of each droplet because it controls the charge quantity of ejected liquid to deflect its flying direction. Therefore, the continuous ink-jet type is simpler than the drop-on-demand type and can assure high reliability. Consequently, the continuous ink-jet apparatus has been widely used as industrial marking apparatus which requires a long service life and high reliability. Details of these ink-jet types and industrial marking apparatus which uses the continuous ink-jet apparatus are explained in detail by Non-Patent Document 1.
As explained by Non-Patent Document 2, many of the continuous ink-jet apparatuses for industrial marking use a single nozzle and control the quantity of deflection of the ejected liquid to form images. However, to use the long-life high-reliability continuous ink-jet apparatus in various fields, the continuous ink-jet apparatus must be of the multi-orifice type that uses a plurality of orifices to eject ink.
To realize a continuous ink-jet apparatus of the multi-orifice type, the most important problem is to form uniform droplets from a plurality of parallel orifices. For this purpose, some methods have been proposed.
Patent Documents 1 to 4 disclose a method of vibrating an orifice plate by piezoelectric elements. However, it is difficult to vibrate the orifice plate uniformly by piezoelectric elements in spite of various contrivances. Because of generation of a mode of plate vibration or a vibration mode, this method has a demerit that droplet generation timing and quantities may vary by the positions of orifices formed in the orifice plate.
Patent Documents 5 and 6 disclose a method of vibrating the whole ink chamber. This method requires much vibration energy as the liquid and the whole ink chamber must be vibrated. Further, it is hard to increase the vibration frequency in this method. Generally, the droplet generation frequency must be some kHz to 10 kHz considering the productivity. At the present time, the droplet generation frequency of the drop-on-demand ink-jet apparatus is about 10 to 20 kHz and that of the continuous ink-jet apparatus is 100 kHz or higher. Further, it is hard to increase the vibration frequency of the ink-jet type that vibrates the whole ink chamber because the vibration load is great.
Patent Document 7 discloses a method of vibrating the orifice plate which contains a plurality of orifices from its side and forming droplets by using propagation of a pressure wave of the liquid. This method using the propagation of a liquid pressure wave cannot generate droplets simultaneously from all orifices and makes the later control such as droplet charge control and droplet deflection control complicated. Further, this method also has a stability problem since the pressure wave propagation path is long and waves reflected on the surrounding walls may have a bad influence on droplet generation.
Patent Document 8 discloses a method of providing piezoelectric elements opposite to the orifice plate and vibrating the liquid thereby. This method is assumed to be able to generate droplets most uniformly since the piezoelectric elements can vibrate the liquid uniformly in parallel with the orifice plate. However, the piezoelectric elements in this method must generate uniform flexible deformation on the orifice plate. Therefore, this makes the structure of the piezoelectric elements very complicated. Further, the quantity of deformation of each piezoelectric element is not so big and a value in nanometers. As this method directly vibrates liquid, this method requires comparatively great deformation of each piezoelectric element. This requires a structural device and greater supply voltages.
Patent Documents 9 and 11 disclose a method of equipping each piezoelectric element with a resonator which is opposite to the orifice plate. The resonator amplifies the vibration force of the piezoelectric element to a vibration force of comparatively great amplitude. Further, this method places the vibrating resonators oppositely to the orifice plate to vibrate liquid uniformly in parallel with the orifice plate. In other words, this method can use very small energy to generate liquid droplets by amplifying a comparatively small displacement of a piezoelectric element by a resonating material. However, this method requires controlling the vibration manner of resonators that vibrate by piezoelectric elements to the desired vibration manner.
[Patent Document 1] U.S. Pat. No. 3,739,393
[Patent Document 2] U.S. Pat. No. 3,777,307
[Patent Document 3] U.S. Pat. No. 6,357,866
[Patent Document 4] EP0943436
[Patent Document 5] EP0461238
[Patent Document 6] U.S. Pat. No. 6,505,920
[Patent Document 7] EP0819062
[Patent Document 8] U.S. Pat. No. 4,520,369
[Patent Document 9] U.S. Pat. No. 6,637,801
[Patent Document 10] WO98/08685
[Patent Document 11] U.S. Pat. No. 5,912,686
[Non-Patent Document 1] “Inkjet Printer Technologies and Materials” edited by Takeshi Amari, published by CMC, 1998